A .download file is created when the download process is interrupted before it completes. Because it isn’t complete, it won’t work to install SketchUp. Try downloading again, and wait until you see only the .dmg extension.
Neither of them have the file name you gave. Whichever one you do get, double-click on the DMG file, then in the window that appears you drag the SketchUp folder on to the Applications alias, to set the folder copying across from the disk image to your drive.
After copying, eject the disk image file, then go into Applications, SketchUp 2017 or 2021, and open SketchUp from inside that folder.